Concern that Lake Oroville had reached capacity, on Saturday morning, for the first time ever, water was directed down an emergency spillway, due to damage to the dam's regular spillway.
Oroville Dam, which is the nation's tallest dam, is located about 25 miles southeast of the city of Chico.
Water began flowing down the emergency spillway at 8 a.m. Saturday, into the Feather River.
A hole, about 250 feet long and 45 feet deep, opened up in the lower part of the main spillway on Tuesday.
The concrete spillway is separate from the dam itself.
Officials do not know yet what caused the hole.
They continued to use the spillway, in order to keep Lake Oroville from overflowing, but the dam releases caused further erosion.
The hole reportedly grew to at least 300-feet wide and 500-feet long.
Dam operators were hoping not to use the emergency spillway, but were preparing for it, just in case.
But because of concerns that water levels were nearing capacity at Lake Oroville, officials turned to the emergency spillway for the first time in the dam's 48-year history.
Officials say Feather River can handle the extra water, and the dam is intact and not in danger of failing.
The state said the city of Oroville, a city of about 15-thousand residents, is not at risk of flooding.